The motion of the respiratory cycle affects tumor sites in the thoracic cavity, abdomen and pelvis. The relative position of target carcinomas of the breast and lung in the thoracic compartment, liver, stomach and pancreas in the abdominal compartment, and prostatic and gynecologic carcinomas within the pelvic compartment are all affected by respiration. Intrafraction target motion, (tumor motion occurring within a treatment session) is an issue that is becoming increasingly important with external beam radiotherapy. Intrafraction motion can be caused by one, or a combination of, the respiratory, musculoskeletal, cardiac, and gastrointestinal systems, with the respiratory system being the dominant cause of tumor motion during treatment. The ultimate goal for dose delivery is to obtain a static target relative to the treatment beam's eye view whenever the beam is on and deliver radiation only to the tumor, sparing the surrounding healthy tissue. Effective dose targeting is becoming increasingly important, as dose-escalation is used in an attempt to improve long-term tumor control and improve patient survival.
Respiratory motion, the most significant contributor to intrafraction motion, can generate artifacts in all imaging modalities. Patient's breathing patterns can vary in magnitude, period and regulation during imaging and treatment sessions. If respiratory motion is not considered during image acquisition, when conventional radiotherapy techniques are used, artifacts are created in the acquired image and can distort the target volume and provide the clinician with incorrect positional and volumetric information of the tumor. For example, with computed tomography (CT) imaging, the artifacts primarily occur because different parts of 3D target structure(s) move in and out of a 2D slice window during the respiratory cycle and time of acquisition.
During radiation treatment planning, margins are allocated to ensure adequate coverage of the tumor and surrounding area for suspected microscopic spread of cancerous cells. Additional treatment margins are also added to accommodate tumor intrafraction and interfraction motion (motion between treatment sessions), as well as patient setup error. Adding treatment margins to cover the limits of tumor motion due to respiration is non-optimal because this increases the radiation field size and thus exposes a volume of healthy tissue to high doses of radiation which can create additional complications.
In an attempt to mitigate respiratory induced intrafraction tumor motion, techniques called “Deep Inspiration Breath Hold” (DIBH) and “Active Breathing Control” (ABC) have been developed. In the DIBH technique, the patient is verbally coached and brought to a reproducible deep inspiration breath-hold level. Active Breathing Control has been used and found to be fairly effective in stabilizing lung volume, and in the case of patients with lung tumors, lung tumor position. In this approach, a spontaneously breathing patient breathes through a valve that is closed depending on a respiratory signal derived from a digital spirometer and held closed while the radiation beam is turned on. However, the ABC and DIBH techniques are only applicable and useful for those patients who are alert, oriented, cooperative and understand the respective coaching instructions required of both methods. The breath-hold method is typically difficult for lung cancer patients, for which respiratory motion management is most critical, as they cannot hold their breath for extended periods due to compromised pulmonary function.